(Reuters) – Denver Broncos wide receiver Wes Welker has been suspended for the first four games of the 2014 season for violating the National Football League (NFL)’s performance-enhancing substances policy.

The league never makes public what substances athletes test positive for but the Denver Post reported it was the banned stimulant Adderall, a drug prescribed to treat Attention Deficit Hyperactivity Disorder (ADHD).

The news comes just five days before the Broncos, who lost last season’s Super Bowl, begin their season with a home game against the Indianapolis Colts.

“Tonight’s news regarding Wes is very disappointing for our team, but we understand the league’s authority in this area,” Denver coach John Fox said on the team’s official website.

“I have no doubt that Wes will remain focused on his preparations for the season and stay in excellent shape during his time away from the team.”

In an email to the Post, Welker said he was “as shocked as everyone” and would fight to clear his name.

“I would NEVER knowingly take a substance to gain a competitive advantage in any way,” he said.

“I have never been concerned with the league’s performance enhancing or drug abuse policies because under no scenario would they ever apply to me.

“But I now know that (drug-policy procedures) are clearly flawed, and I will do everything in my power to ensure they are corrected, so other individuals and teams aren’t negatively affected so rashly like this.”

Welker, who has been to the Pro Bowl five times, is a key receiver for Denver quarterback Peyton Manning and made 778 yards on 73 receptions last year.

(Reporting By Andrew Both in Cary, North Carolina, editing by Nick Mulvenney)